Despite intense development of new anticancer substances, the clinical treatment of most frequently diagnosed solid tumors needs to be improved and for some malignancies reasonably efficient therapies need to be developed, as they are practically non-existent. Early detection followed by surgery remains the main tool that enables significant expansion of life span for majority of patients. In most malignancies it may be necessary to modulate (preferably in a synergistic manner) several relevant biological pathways. Accordingly, the required phenotype (death of tumor cells) can be elicited by synthetic lethal modulation of properly chosen biological processes. Synthetic lethal interactions tend to form clusters; one significant network of such interactions encompasses the biological processes involved in the DNA damage/repair. Selective and efficient activity modulation of selected processes is therefore of significant importance and can lead to a new generation of modern anticancer drugs.
Maintenance of genomic integrity ensured by multifaceted cellular DNA damage response (DDR) is a fundamental biological phenomenon shared by all organisms. On one hand, the DDR network of genome surveillance, checkpoint and repair pathways counterbalances the potentially mutagenic effects of endogenous (oxidative and replicative lesions) and exogenous (e.g. ionizing or UV radiation, cigarette smoke) DNA damaging assaults. On the other hand, modulation of selected components can be exploited in efficient treatment of malignant diseases. It is likely that optimal synthetic lethal treatments will be different for particular tumor sub-populations; this approach is therefore compatible with the concept of personalized medicine.
Mammalian MUS81 protein with its partners EME1 or EME2 form a heterodimeric structure-specific endonuclease that preferentially cleaves 3′ flaps and replication fork intermediates (Genes Dev. 2001, 15, 2730.). This endonuclease has been shown to facilitate restart of stalled DNA replication forks by generating DNA double-strand breaks (EMBO J. 2006, 25, 4921.). MUS81 also interacts with other DNA damage repair proteins including Rad54, BLM, as well as SLX4 (Cell 2009, 138, 78.; Mol. Cell 2009, 35, 116.; Cell 2009, 138, 63.). In addition, inactivation of MUS81 has been shown to result in chromosomal abnormalities and increased sensitivity to crosslinking agents (Nucleic Acids Res. 2006, 34, 880.; Science 2004, 304, 1822), indicating essential role of Mus81 in genome maintenance. Accordingly, decreased levels of MUS81 expression have been found in hepatic metastasis and correlated with poor cancer prognosis (Cancer 2008, 112, 2002). Furthermore, it has been recently shown that dual inactivation of CBX2 and MUS81 remarkably affect cancer cells (PLoS Genet. 2011, 7, e1001385.). These findings suggest that MUS81 is a good target for pharmacological intervention.
Amongst all DNA repair processing enzymes, the MRE11-RAD50-NBS1 (MRN) complex plays an important role in preserving genomic integrity by acting as a DNA damage sensor of double strand breaks (DSB) and by promoting repair through non-homologous end-joining (NHEJ) or homologous recombination (Nature Reviews 2002, 3, 317.; Trends Biochem. Sciences 2002, 27, 410.). In response to DSB, MRN activates and recruits ATM (belonging to the phosphatidylinositol-3′ kinase-related kinases (PIKKs) family) to damaged DNA sites. ATM initiates a signaling cascade leading to cell cycle arrest and DNA repair. MRE11 is the subunit core of the MRN complex and displays 3′-5′exonuclease activity, single-stranded and DNA-hairpin endonuclease activity. The MRE11-RAD50 complex functions include DNA binding, bridging the ends of DSBs and their processing. NBS1 does not possess any enzymatic activity; its role lies in signaling and interacting with other proteins (DNA Repair 2010, 9, 1299.; Cell 2008, 135, 97.). The significance of MRN complex is underlined by the fact that germline mutations of MRE11, NBS1 and RAD50 cause ataxia-telangiectasia-like disease (ATLD), Nijmegen breakage syndrome (NBS) and NBS-like disorder (NBSLD), respectively (Cell 1998, 93, 477.; Cell, 1999, 99, 577.; Am. J. Hum. Genet. 2009, 84, 605). ATLD, NBS and NBSLD have similar features as does ataxia-telangiectasia (AT), caused by mutations in the ATM gene, which include hypersensitivity to DSB-inducing agents, chromosome fragility, DNA damage-dependent cell-cycle arrest and high predisposition to cancer (Cell 1998, 93, 477.; Oncogene 2007, 26, 7749; Cell 1999, 99, 577.; Am. J. Hum. Genet. 2009, 84, 605.). In addition, depletion of MRE11 leads to sensitization to poly(ADP-ribose) polymerase (PARP) inhibition (Cancer Res. 2011, 71, 2632.). Futhermore, MRE11-deficient cells are also sensitive to topoisomerase poisons, suggesting a role of MRE11 in removal of TOP1/TOP2-lessions and in stimulating an effect of topo inhibitors (Mol. Cell. Biol. 2004, 24, 9682.). Indeed, triapine (RNR inhibitor) was recently shown to block MRN-mediated recombination and sensitize ovarian cancer cells to PARP and topo inhibitors (Mol. Cancer Res. 2014, 12, 381.; Cancer Res. 2012, 72, 2814.). Therapeutic importance of MRE11 inhibitors in modern oncology is further supported by recently reported synthetically lethal genetic interactions for MRE11-FEN1 (PLoS Genet. 2013, 9, 1, e1003254.) and MRE11-BRCA2 (Cancer Res., 2012, 72, 2814.).